1. Field of the Invention
The present invention relates to zirconium oxide hydrate particles that contain a large amount of water of hydration and exhibit high proton conductivity, and to a production method therefor.
2. Description of Related Art
Aluminum oxide, doped cerium oxide, zirconium oxide, various composite oxides and like inorganic materials are inorganic proton-conductive materials, and are applicable to a variety of uses in, for example, solid acid catalysts, electrolyte materials for use in electrochemical capacitors, fuel cells and the like, various gas sensors such as hydrogen sensors, oxygen sensors and the like, etc. Among such inorganic proton-conductive materials, zirconium oxide is superior in terms of ease, safety and like characteristics in the production process thereof, and is a material used highly frequently.
As an application of zirconium oxide and the like, for example, JP 2004-325388A proposes a gas sensor containing a gas sensitive material composed of a solid super strong acid substance containing titanium oxide, zirconium oxide, tin oxide or the like. Moreover, JP 2000-19152A discloses that stabilized zirconia or stabilized ceria is effective as an ion-conductive electrolyte for hydrogen gas sensors. Furthermore, JP 2002-83611A proposes a solid electrolyte fuel cell containing a solid electrolyte layer composed of stabilized zirconia or a ceria-based oxide.
Such zirconium oxide is present also as a hydrate. Production methods therefor that are generally employed include a method using a neutralization reaction between an alkali solution and a zirconium salt solution and a method using a hydrolysis reaction of a zirconium salt by ammonia. The amount of water of hydration (n) of the zirconium oxide hydrate represented by the formula ZrO2.nH2O produced according to these methods is only about 2.5 when in a dried state at room temperature. Here, the water of hydration of the zirconium oxide hydrate includes both the water of adsorption that is adsorbed on the surface of zirconium oxide hydrate particles and the water of crystallization present in the crystal of zirconium oxide hydrate particles.
In addition, as a method for producing fine zirconium oxide particles, JP 2006-143535A proposes a method for producing a zirconia sol in which fine zirconia particles have a mean particle size within the range of 5 to 100 nm according to the neutralization-precipitation method. Moreover, JP 2005-170700A proposes also a method for producing zirconium oxide particles having a mean particle diameter or a mean particle major-axis length of 1 to 200 nm according to the neutralization-precipitation method.
It is known that, generally, the larger the amount of water of hydration a zirconium oxide hydrate has, the higher the proton conductivity thereof. A zirconium oxide hydrate having a large amount of water of hydration ideally (1) is of ultrafine particles so as to have a large amount of water of adsorption with an increased specific surface area, and further (2) needs to have low crystallinity so as to contain large amounts of both water of crystallization and water of adsorption. Thereby, the amount of water of hydration as a whole obtained as the total sum of water of adsorption and water of crystallization can be as large as possible. Thus, to obtain a zirconium oxide hydrate having high proton conductivity, the particle size of the zirconium oxide hydrate should be as small as possible and the crystallinity thereof should be as low as possible.
However, in order to make the particle size of a zirconium oxide hydrate small, uniform fine particles having high dispersibility need to be obtained by increasing the crystallinity. However, increasing crystallinity creates a problem in that the amounts of water of adsorption and water of crystallization are both, decreased. On the other hand, when the crystallinity of a zirconium oxide hydrate is lowered, zirconium oxide hydrate particles have a strong tendency to adhere to each other with a small amount of heat regardless of whether in a wet atmosphere or a dry atmosphere and form coarse particles, thereby making it difficult to obtain uniform fine particles. As a result, although the amount of water of adsorption and the amount of water of crystallisation are increased to some extent due to the decrease in crystallinity, the specific surface area is decreased due to the increase in particle size, thereby imposing a limit also to the increase of the amount of water of adsorption.
Thus, it has been difficult to obtain zirconium oxide hydrate particles having a large amount of water of hydration that have low crystallinity and are of ultrafine particles.